The present invention relates to an electrically operated power steering device for giving steering auxiliary force to the steering mechanism of a vehicle by driving an electric motor in accordance with steering torque applied to an operation unit for vehicle steering, and more particularly, relates to a voltage raising circuit for generating a voltage to be applied to the electric motor by raising a battery voltage in such an electrically operated power steering device.
The electrically operated power steering device for giving the steering auxiliary force to the steering mechanism by driving the electric motor in accordance with steering torque applied to a handle (steering wheel) by a driver is conventionally used. In this electrically operated power steering device, a torque sensor for detecting the steering torque applied to the handle as an operation unit for steering is arranged, and a target value of an electric current to be flowed to the electric motor is set on the basis of the steering torque detected by this torque sensor. A command value to be given to a driving unit of the electric motor is generated on the basis of the deviation between this target value and the value of the electric current really flowed to the electric motor. For example, the driving unit of the electric motor is constructed by a PWM signal generating circuit for generating a pulse width modulating signal (PWM signal) of a duty ratio according to this command value, and a motor driving circuit constructed by using a power transistor turned on and off in accordance with the duty ratio of this PWM signal. The voltage according to this duty ratio, i.e., the voltage according to the command value, is applied to the electric motor. The electric current flowed to the electric motor by this voltage application is detected by an electric current detector. The difference between this detecting value and the target value is used as the deviation for generating the command value. In the electrically operated power steering device, feedback control is performed such that the electric current of the target value thus set on the basis of the steering torque is flowed to the electric motor.
In such an electrically operated power steering device, a construction for further adding a voltage raising circuit for raising and supplying the voltage of the battery is known. Various constructions are considered in this voltage raising circuit. FIG. 9 is a circuit diagram showing one example of the voltage raising circuit. This voltage raising circuit includes an oscillating circuit 40 for inputting a control signal thereto from the exterior and outputting a pulse wave according to this control signal, a transistor Tr41 to which this pulse wave is input and which performs a switching operation according to this pulse wave, a coil L41 for repeating the accumulation and discharge of energy by this switching operation, and capacitors C41, C42. This voltage raising circuit makes the transistor Tr41 perform the switching operation by the pulse wave from the oscillating circuit 40, and the energy in the coil L41 is repeatedly accumulated and discharged by turning on and off the electric current flowed to the coil L41. Thus, a high voltage is repeatedly generated on the cathode terminal side of a diode D41, and a smoothed raised voltage is generated by the capacitor C42. In accordance with such a construction, larger torque is obtained by applying the raised voltage to the motor.
A voltage drop in the forward direction in the diode used in the voltage raising circuit is set to 0.6 to 0.7 V and is large. Therefore, a voltage raising circuit using a MOS (metallic oxide semiconductor) type transistor (hereinafter simply called “MOS”) having a small voltage drop instead of the diode is known. In such a circuit construction, the MOS is replaced with the diode by making a source terminal and a drain terminal of the MOS correspond to an anode terminal and a cathode terminal of the diode D41, respectively, in the voltage raising circuit (see a main voltage raising section 31 of FIG. 5 described later with respect to a constructional example). If this MOS is operated by giving a pulse wave to the gate terminal of the MOS so as to be reciprocally turned on and off with respect to the turning on and off of the transistor Tr41, similar to the case of the diode, a high voltage can be repeatedly generated on the drain terminal side of the MOS.
However, since the raised voltage is basically supplied at any time in the voltage raising circuit used in the electrically operated power steering device, switching loss, etc. based on the switching operation of the transistor are always generated. As this result, a problem exists in that energy loss is increased.
When an ON-period of the MOS becomes longer than a predetermined time in the voltage raising circuit using the MOS instead of the diode, the electric potential on the drain terminal side becomes higher than that on the source terminal side. Therefore, the phenomenon (hereinafter called a “back flow phenomenon”) that the electric current is flowed backward from the drain terminal side to the source terminal side, is caused. A problem exists in that heat is generated in the MOS and voltage raising efficiency is reduced by this back flow phenomenon.
Further, for example, when the motor is suddenly moved by external force by dropping a wheel to a groove, etc., the motor attains an electricity generating state. When the MOS is turned off at this time, a problem exists in that electric current to be fed back to the battery is not ted back and the danger of internal damage is caused.